You get 'experience' by playing the game more as each race, which will unlock more
hats portraits in the game, as well as display your level on your profile.
Seems like Blizzard is trying to get into the reward for playing model that many free-to-plays have (LoL, airmech). But the rewards are purely cosmetic/reputation based (unlike CoD, Battlefield). I'm kinda torn on this. I think it can be dangerous to introduce artificial incentives in a skill based game like Starcraft, that could easily result in people playing differently than normal. On the other hand, I like the idea of introducing more profile customization and tying that to playing the game.
I can imagine a scenario where a player ends up wasting his/her opponent's time by staying in the game longer just to get more experience, which could potentially be very annoying.